‘Sept- 23, 1958
A J. CATOTTI ET AL I
‘
PROCESS. OF ETCHING ALUMINUM FOIL.
FOR ELECTROLYTIC CAPACITOR‘
Filed April 6. 1956
2,853,445
/6
ATTORNEYS
United States Patent
C6
2,853,445
Patented Sept. 23, 1958
1
or scaling, as above pointed out, and this may materially
offset the cost advantage such a process has over elec
2,853,445 ,
trochemical etching methods.
ELECTROLYTIC CAPACITOR
Arthur J. Catotti, Marion, Hans Cohn, New Bedford,
and Peter P. Grad, South Dartmouth, Mass., assignors
it]: Aerovox Corporation, a corporation of Massa
usetts
-
-
‘ >
>
'
Because of the increased rate of etching that has been
found to accompany the ‘presence of greater amounts
PROCESS OF ETCHING ALUMINUM FOIL FOR ‘ '
of impurities in aluminum foil, it is generally believed
that some of these impurities act to catalyze the etching
process. This theory is further borne out by the fact
10
that conventional chemical etching processes are relatively
ineffective in the etching of high purity aluminum foil,
that is, foil containing at least 99.99% aluminum. ‘Etch
ing of such high purity aluminum foil can be carried out
8 Claims. (Cl. 204-441)
only by chemical methods that maintain the tempera
ture of the electrolyte at an undesirably high level,_such
This invention relates to the etching of aluminum foil 15 as'in the. order of 100° C., or by the useof relatively ex- "
Application April 6, 1956, Serial No. 576,673
pensive electrochemical etching methods.
and more particularly to a process adapted to the etch
ltlis an object of the present invention, therefore, to
provide a process for the etching of aluminum foil, which
is materially less sensitive to small variations in the im
ing of both high purity aluminum foil (i. e. having a
purity of 99.99% or higher) and to other commercial
aluminum foils containing greater amounts of impurities.
The etching of the surface of aluminum foil'as a step 20 purity content of the aluminum than are conventional
chemical etching processes, without involving the expense
for equipment and operation of conventional electro
to the art. Normally smooth aluminum foil is etched
in the production of electrolytic capacitors is well known
to produce a microscopically roughened surface presenting a substantially greater area than that of the smoothfoil.
Subsequent to etching, the foil undergoes an anodizing
treatment whereby a ?lm of aluminum oxide is formed on
the foil. Greater surface areas of aluminum will naturally
produce greater areas of interfacial contact between the
chemical etching processes.
. I
Another object of the invention is to provide a chemical I
25 etching process for use with high purity‘aluminum foil
which can operate at relatively low and easily controllable
temperatures.
-
These and other objects of the invention, which will
be in part speci?cally pointed out and in part apparent
aluminum and the oxide ?lm, which, in turn,‘ is desirable
in that the capacitance of a condenser made'in this way 30 from the subsequent description, are attained by a gal.
vanic etching process in which an electrical conductor
is directly proportional to this area of interfacial. contact.
is connected between the aluminum foil to be etched and
Such etching processes have been carried out both chem
an electrode of an'element more noble or more electro
ically and electrochemically with the chemical method
positive than aluminum while maintaining the'foil and
being more widely used because of the smaller initial
investment required to install the equipment. The usual 35 the electrode in contact with an electrolyte comprising
an aqueous solution of an acid and a 'salt that‘has a
chemical etching process is carried out by immersing the
cation which is not in its lowest state of oxidation and
aluminum foil in a dilute aqueous solution of a highly
which‘ is capable of forming a soluble salt in a 'lower
ionized mineral acid, such as hydrochloric acid, for a
oxidation state. The aluminum is oxidized to‘ form va
predetermined period. The process has been re?ned in
various‘ ways, such as by the addition of various catalytic 40 solution of aluminum salt of the acid while'the other
salt is reduced to its lower oxidation ‘stage. 'In such a
agents, such as aluminum chloride, to the etching bath.
process no insoluble reduced product is formed that must
Chemical etching processes, however, have proved diffi
be disposed of and the solution can be renewed from time
cult to control in normal manufacturing operations. It
to time by addition of further amounts of reducible salt. .
has been found that a very minute deviation in the amount
of impurities contained in the aluminum foil to be etched 45 In a preferred embodiment of the invention, the elec—
trode is of carbomwhich, while conducting electricity,'is '
,causes very substantial di?erences in the amount of etch
‘electrochemically inert and has no tendency to react with
ing obtained and in the rate of etching. For example,
a'chemical etching procedure that is satisfactory. for
aluminum foil of 99.85% purity, would, in ‘many cases,
the electrolyte. In conjunction with this electrode a pre- '
a case often accelerates the etching process to such an
ride which remains in solution in the ‘electrolyte’. “The
ferred electrolyte is made up of an aqueous solution of
be totally useless for a foil of 99.80% purity. Thedif 50 hydrochloric acid and ferric chloride. Duringthe etch
ing process,'the aluminum is oxidized to aluminum chlo
ference of 0.05% in the amount of impurities in such
use of ferric chloride is’ ofparticular advantage since this
extent that ‘the etched solution burns holes in the foil
solution can be regenerated to ferrous chloride simply by
or produces a surface coating of loose scales. On vthe
other hand, the use of a slightly 'purer foil, for example, 55 blowing air through the spent solution. ' The carbon elec
trode, being inert, has no tendency to dissolve in ‘the acid
one having an aluminum content of 99.90%, would’ re
solution, as would a metal electrode, even though the
sult in such a' sharp decrease in the rate of etching'that
metal were below aluminum (but above hydrogen) in the
an undesirably small gain in surface area would result
electromotive series. Thus, no reaction takes place tend~
fromthe etching process. I
'
'
1
ing to oppose the main reaction, i. e.
I _'
.From the above it will be apparent thatlto carry out
alconventional etching process extremely close and ac
curate control must be maintained in purity of the alumi
Instead of an inert electrode, it is sometimes more con
venient to employ a metal electrode composed-of an .ele~
this‘ reason, substantial amounts of aluminum foil are 65 ment below aluminum in the. electromotive series. A pre
oftenlost to’. the capacitor manufacturer-due. to burning ~ .ferred electrode .of this class is iron because it, is inex
num'foil used; Suchcontrol is not always possible due
to 'variations’in large .scale production of foil. ‘For
2,853,445
3
.
pensive and readily available. However, chromium, co
balt, lead, copper, silver, platinum and gold may also be
employed as electrodes, provided they do not contaminate
the particular etching solution employed by entering into
side reactions with it.
The more noble metals are more
'4
out deter-mines its effectiveness as measured in terms of
so-called “capacitance yield.” Capacitance yield is the
61
effective because of the greater electromotive force set
up, but this advantage is more than offset by the high
price of such materials.
,
The temperature at which galvanic etching is carried
number of square inches of foil necessary to produce one
microfarad of capacitance after the foil has been formed
with an aluminum oxide ?lm. In order to effect economy,
it is naturally desirable that this capacitance yield be as
low ‘as possible. It has been found that in the galvanic
etching of high purity aluminum foil, it is possible to ob
‘In addition to pure metals or
elements, alloys may also ‘be employed and the term “ele
10 tain substantial etching and satisfactory capacitance
ment” as used hereinafter is intended to include alloys.
The ‘process of the present invention is illustrated by a
yields at much lower temperatures than can be employed
single ?gure of drawings which show diagrammatically
using conventional chemical etching methods. At tem
apparatus for carrying out the etching process.
peratures in the range of about 20° C. to about 80° C.
7 Referring now speci?cally to the drawing, there is in
it has been found that capacitance yield improves as the
dicated generally at 10 a tank, containing an electrolyte
temperature increases to a maximum in the neighborhood
11, into which is fed the aluminum foil 12 to be treated,
of about 80° vC., the capacitance yield tending to become
which foil passes through the tank under the submerged
poorer with further temperature increase. In practice a
roller 13 and emerges over the roller 14. The speed with
substantially constant temperature of the etching bath of
which the aluminum strip 12 is fed- through the tank
between about 30° C. and about 80° C. is preferred.
10 is regulated so as to provide for a controlled period of 20
Table I, below, illustrates the variation of capacitance
immersion in the electrolyte 11. As the strip 12 enters the
yield with etching temperature. Three different values
are given for capacitance‘ yields, i. e. the yields obtained
tank it comes into contact with an electrical connector,
such as brush shown at 15, which is connected to a con
by forming the oxide ?lm at 100, 300 and 600 volts. The
ducting wire 16. Also immersed in the electrolyte is the
oxide ?lm is formed at the different voltages depending
' electrode 17 which is connected to the brush 15 by means
upon the maximum voltage to which the capacitor is ulti
of the wire 16. The electrode 17 is made of some ele
mately to be subjected. The data set forth in Table I,
as Well as the subsequent tables, was obtained by the
ment below aluminum on the electromotive series and is
following etching procedure:
preferably made of carbon or desirably of metallic iron,
although lead, copper or other preferably inexpensive ele
The foil is ?rst washed for 30 seconds in a 2% aqueous
30 solution of sodium hydroxide maintained at about 82° C.
ment could be used in place of iron, if desired.
The sodium hydroxide is then removed by washing in
The electrolyte 11, through which the aluminum foil
12 is passed, is made up of an aqueous solution of a
distilled water and the foil is etched in the manner above
. strong mineral acid and a salt of a metal that has at least
set forth. Following etching, the foil is again washed
in distilled water, followed by washing in dilute nitric
two oxidation stages, which salt is soluble in each of its
oxidation stages. Among the metals whose salts possess
this property are iron, copper, nickel, cobalt, chromium
acid maintained at about 65° C. for about 30 seconds.
Following this acid treatment, the foil is again washed
and then boiled in distilled water and then dried and sub
sequently oxidized and tested. In Table I, below, all of
the samples were subjected to ?ve minutes of galvanic
and tin among others, of which iron is generally pre
ferred. While any soluble salt would be suitable for this
purpose, such as the nitrates, sulphates, or the like, the
chlorides are generally preferred. Thus, the chloride of 40 etching in a solution of 1.09 N I-lCl and 0.135 N FeCl3.
A soft iron anode was employed.
iron is generally to be used.
The nature of the acid employed also admits of sub
stantial variation. It should be understood that in the
TABLE I
process of the present invention, the function of the acid
is not to attack the aluminum dire-cly as in conventional 45 E?ect of temperature on capacitance yield of 99.99% foil
etching. The aluminum is dissolved by a reaction with
(F~f0i l)
a reducible metallic ion, such- as the ferric ion. The acid
merely serves to prevent precipitation of basic salts of
Capacitance Yield, Sq.
the reducible metal. Thus, any acid may be employed,
Etch
Percent
In./Mfd.
Sample
Temp,
Wt.
although mineral‘ acids, such as hydrochloric acid, are 50
° 0.
Loss
preferred for economic reasons. Preferably, the acid
100
300
600
and the reducible salt should have the same cation so
Volts
Volts
0. 95
0. 95
0. 97
0. 56
0. 41
0. 27
0. 17
0. 20
2. 63
2. 63
2. 56
1. 96
1. 24
0.87
0. 68
0. 68
Volts
that the etching process may be aided by the common ion
e?ect.
28. 0
28.0
30. 3
50. 8
62. 5
70. 3
78. 3
80. 0
'
'The concentration of the acid and salt in the electrolyte
is not believed to be particularly critical to the operation‘
of the etching process. Since the acid does not function
as a direct etchant, its concentration may be substantially
lower than that used in conventional etching solutions.
Thus, by employing only enough acid to prevent forma
tion of insoluble salts, the desirable result is attained of
minimizing the gaseous hydrogen formed by the direct
reaction of aluminum with acid.
A convenient concen
tration of hydrochloric acid has been found to be 1 N,
although concentrations as low as .01 N and as high as
2 to 3 N may be employed.
‘Concentration of the ferric chloride is determined
primarily by the amount of etching desired since three
19. 8
17. 2
17. 8
17. 7
23.0
25. 8
29. 8
30. 0
5.14
5. 14
5. 14
4. 00
2. 92
2. 45
1. 72
1. 96
60
The elfect of etching time on capacitance yield has been
found to be somewhat similar to that of etching tempera
ture in that there appears to be an optimum time. In
Table II, below, there is illustrated the results of etching
various grades of aluminum foil galvanically for vary
ing periods. In Table II, three types of foil were used.
The samples indicated as “B” foil were 99.80% aluminum.
The “C” samples were 99.85% aluminum. The “F”
ferric ions are used for every aluminum ion formed. The 70 samples were high purity aluminum foil containing 99.99%
concentrations of the acid and ferric chloride are pref
aluminum of the same types employed in Table I. A
erably about the same order of magnitude, although there
number of samples of foil were etched galvanically, using
appears to be no critical relation between the concen
‘ tration of these constituents. Thus, the ferric chloride
an iron anode in a solution of 1.05 N HCl and 0.185 N
concentration may vary between about .01 N and 3' N.
FeCl3 at 45° C. The results given in Table II, below,
75 indicate that for both the “B” and “C” foil, improvement
2,853,445
6
5
TABLE .v‘
Galvanic etch at 45° C.
in capacitance yield is associated with greater etching times
up to about 12 minutes, and for the “F” foil up to about
14 minutes.
TABLE 11
Capacitance Yield,
Sample
Capacitance Yield, Sq.
Etch
Sample
Time,
Min.
Percent
' 0
Min.
Sq. In./Mid. @
Wt. Loss
100
In./Mid. @
300
600
Volts
-
Volts
Volts
0.719
0. 781
0.473
0. 517
0.398
0.442
2.18
2.34
1. 30
1. 50
1. 13
1. 36
Loss
15.1
300 '
Volts
Volts
600
1. 34
3. 40
6. 64
’ 8.85
. 62
2.10
4. 00
4
11.9
.392
1.28
2.70
6
13. 7
. 284
1. 13
8
18. 5
. 274
. 784
10
28. 8
.235
.89
1.82
33.1
. 233
.74
1.57
15
24. 2
. 215
. 784
1.74
______ _ _
2. 90
6. 3O
2
12. 7
. 535.
1. 73
3. 76
4
6
17. 5
21.0
.442
.348
1.33
1.15
2. 90
2. 43
8
24.0
.256
. 815
10
32. 5
. 226
. 675
1. 63
10
10
v 12
22.8
28.4
30.6
. 20
.22
.176
.79
. 79
.70
l. 70
1. 67
1.60
14
31.8
0
10. 4
4
6
8
17.1
19. 3
20.0
.20
______ ..
. 605
. 680
. 475
8. 4
7. 2
12.4
10. 0
l4. 6
11.3
4. 46
4. 53
2. 84
2. 98
2. 39
2. 88
2. 46
2. 06
12
l1. 3
3
3
6
6
9
9
Volts
2
0
Time, Percent
.
100
p
Etch
1. 87
.69
1.62
2. 48
5. 95
2.10
1. 85
1. 56
4. 25
4. 00
3. 45
10
23. 8
.416
1.41
3.13
12 -
27. 4
. 435
1. 41
3.02
13
14
27.2
28. 1
.416
.325
1.18
1.05
2. 70
2. 42
17
32. 6
. 342
l. 10
2. 70
20
33. 6
. 290
1. 01
_ 2. 24
15
The importance of these tables isnot in a comparison
of the capacitance yields obtained by the. galvanic etch- .
ing as opposed to conventional chemical etching, but
rather in the divergencies of values obtained for “B” foil
20 and “C” foil etched by the two different processes. When
etched by the conventional chemical method, there is a
very substantial di?erence between the capacitance yield
obtained for “B” foil and that obtained for “C” foil,
despite the identity of the etching conditions. For ex
25 ample, from Table III, it will be noted, that the ca
pacitance yield obtained for “C” foil was at some voltages
over 100% higher than that obtained for “B” foil. This
means that a difference of impurities content of only
0.05% results in a tremendous difference in capacitance
30 yield when conventional chemical etching is used.
This
means that a small deviation from the speci?cation of
the foil for which the process is adapted will’ result in
tremendously different electrical properties of the formed
?lm.
On the other hand, Table V shows that when the
tion over more conventional chemical etching methods is
that it etches aluminum foil to an extent that is less de 35 etching process of the present invention is employed,
the same difference of impurity content results in only a
pendent on the amount of impurities present. This is
relatively minor divergence in capacitance yield. Thus,
shown by comparison of the results of Tables III and IV,
a small deviation from the speci?cation of the foil will
below, with Table V, below. Tables III and IV show the
not ‘have the wide divergence in capacitance of the re
results obtained by etching two types of aluminum foil
in a conventional etching bath consisting of an aqueous 40 sultant capacitor which is inherent‘where conventional
One of the principal advantages of the present inven
chemical etching processes are used.
solution of 2.8 N HCl and 0.52 N AlCls for 95 seconds at
60° C. and 65° C., respectively. Table V shows the re
sults obtained by etching the same two types of aluminum
In all of the examples heretofore given, the galvanic
etching of the present invention has been carried out
employing an iron anode. ' However, it has been found
foil galvanically with an iron anode, according to the
that
other elements beside iron are suitable for this pur
45
present invention, in an aqueous etching solution hav—
pose, provided that they are below aluminum in the
ing a concentration of 0.78 N HCl and 0.185 N AlCl3
electromotive series. This includes also those elements
at a temperature of 45° C. for the times thus indicated
which are electrochemically inert, i. e. have no tendency
on the table. In these tables the results are averages taken
to form ions, but which are su?‘iciently good electrical
from numerous samples.
conductors to complete the galvanic circuit. One ‘such
inert element that is particularly preferred is carbon.
TABLE III
Carbon anodes yield particularly bene?cial results when
employed in conjunction with a low acidity etching solu
Chemical etch at 60° C.
tion. Even a metal below aluminum in the electromo
tive series (such as iron) has a tendency to react with
Capacitance Yield,
Sq. InJMfd. @—
Sample
Percent
Wt. Loss
B (99.8% pure Al)‘____
O (99.85% pure Al)---
10. 2
4. 57
the acid etching solution, thus setting up an electro
chemical force opposite to the direction of the over
all galvanic etching reaction, i. e.
'
'
100
300
600
Volts
Volts
Volts
0. 35
0.90
1.14
2. 68
2. 67
5.37
60
Al+3Fe+++->Al++++3Fe++
The results obtained by the etching of high purity
(99.99% pure) aluminum foil in conjunction with a
carbon anode are set forth in Table VI, below. In this
case the foil was immersed for various times in a solu
TABLE IV
65 tion of 0.110 HCl and 0.195 FeCl3 at a temperature of
Chemical etch at 65° C.
Sample
Percent
45° C.
The results set forth in Table VI ‘below may be con
Capacitance Yield,
Sq. Ira/Mid. @
Wt. Less
B (99.8% pure Al) ________________ __
C (99.85% pure Al) _______________ -.
53. 3
15. 6
100
Volts
800
Volts
0. 19
0.252
0.65
0. 83
600
Volts
1.59
1. 81
trasted with those obtained by etching the same high
purity aluminum foil by- a conventional method at tem
70 peratures of about 60° C. using as an etchent an aqueous
solution 2.8 N HCl and 0.52 N A1Cl3 at etching which
was 95 seconds, so that these samples are comparable
with those etched galvanically for 11/2 minutes and re
corded in Table VI below. A comparison of the results
75 of Tables VI and VH clearly illustrates that the galvanic
2,858,445
8
TABLE VI
Time ‘in Min.
Capacitance Yield Av~
Number Wt. Loss erage, Sq. In./Mfd @
of
Samples
6
5
6
5
5
4
vg.,
Percent
________ _.
3. 6
'4. 8
10. 4
21.1
23. 3
100
300
600
Volts
Volts
Volts
1.14
2. 89
5. 20
0.77
0.56
0. 28
0.22
0. 18
2. 38
1. 51
0. 88
0.62
0. 6O
4. 49
3.17
1. 74
1.28
1. ‘i6
2. The process of etching aluminum foil for use in
an electrolytic capacitor, said foil containing small
amounts of impurities, wherein variation in the degree
of etching obtained due to variation of the amount of
said impurities is reduced, comprising introducing the foil
into an aqueous solution of hydrochloric acid and ferric
vchloride, electrically connecting Said foil to an electrode
exteriorly of said solution and insoluble in said solution
and progressively passing said foil through said bath out
10 of contact with said electrode to etch the surface of said
foil.
3. The process according to claim 2 wherein said elec
trode comprises iron.
etching of the present invention is much more effective
4. The process according to claim 2 wherein said elec
than conventional etching, even when using substantially
trode comprises carbon.
lower temperatures and more dilute etching solutions than
those of Table VII.
5. The process according to claim 2 wherein the con
centration of said hydrochloric acid is from about 0.1
N to about 3 N and of said ferric chloride is about 0.1
N to about 3 N.
Conventional etching at 60° C. 2.8 N HCl and 0.52 20
6. The process according to claim 2 wherein the tem
TABLE VII
N AlCl3
Capacitance Yield In
Sample
Sq. InJMfd.
Percent
Wt. Loss
l. 7
1. 5
1. 5
100
Volts
300
Volts
0. 90
1.11
1.05
3. 27
3. 22
3.17
600
Volts
6. l5
6. 35
5. 97
Although the present invention has been described with
respect to a number of speci?c embodiments thereof, it
should be understood that it is not limited to such em
bodiments but embraces also those reasonable equivalents
occurring to those skilled in the art.
What is claimed is:
1. The process of etching aluminum foil containing
small amounts of impurities, wherein variation in the
degree of etching obtained due to variation of the amount
perature of said bath is maintained between about 20°
C. and about 80° C. and said foil is maintained in contact
with said electrolyte for not longer than about 12 minutes.
7. The process of etching aluminum foil containing at
least 99.99% by weight of aluminum prior to anodiz
ing said aluminum for use in an electrolytic capacitor,
comprising the steps of immersing at least a part of a
carbon electrode in a bath of an electrolyte comprising an
aqueous solution of hydrochloric acid and ferric chloride,
establishing electrical connection between said electrode
and said foil exterior of said bath and progressively pass
ing said foil through said bath out of contact with said
electrode to etch the surface of said foil and subse
quently washing said aluminum foil.
8. The process according to claim 7 wherein the tem
perature of said bath is maintained between about 20° C.
and about 80° C.
References Cited in the ?le of this patent
of impurities in said foil is reduced, comprising the steps
of connecting an electrical conductor between said foil
and an electrode spaced therefrom, said electrode com
prising an element more noble than aluminum and insolu
ble in the etch solution, while maintaining said foil and
said electrode in contact with said etch solution, said
UNITED STATES PATENTS
2,052,962
2,162,789
2,336,846
2,699,382
solution comprising an aqueous solution of a mineral
acid and a salt of said acid, the cation of said salt being
in the higher of two soluble oxidation states.
Booe ________________ __ Sept. 1,
Raub ________________ __ June 20,
Clark ______________ __ Dec. 14,
Altenpohl ___________ __ Ian. 11,
1936
1939
1943
1955
FOREIGN PATENTS
854,921
France _____________ .__. Ian. 29, 1940